As an important measurement to reduce carbon emission, electrification of transportation system is an irreversible developing trend in modern society. Battery technologies and related charging methods are key to promote the electric vehicle in the market. With more and more grid-tied battery chargers put in operation, pollution to the AC grid system should be depressed as much as possible through power factor correction. The power factor is defined as the ratio of the real power flowing to the load to the apparent power in the circuit, and is a dimensionless number in the closed interval of −1 to 1. An AC system with a lower power factor draws more current than a system with a higher power factor when a same amount of power is transferred. Generally, the power factor is desired to be 1 for any grid-tied AC system.
FIG. 1 illustrates a schematic of a conventional AC/DC charger circuit with power factor correction. It contains a diode rectifier circuit, a high-frequency filter, a boost DC/DC converter, a stable DC-link capacitor and a DC/DC converter. By using the diode rectifier, the AC voltage vac is rectified to a DC voltage Vin, which has a waveform of the absolute value of the input AC voltage vac. The subsequent boost converter includes an inductor Lb, a switch Qb, and a diode D. By controlling the ratio of the on-time to the off-time of the switch Qb, two objectives are realizable: (1) the variation of the current ib flowing in Lb can follow the change of Vin and be in phase with Vin; and (2) at the same time, the voltage across Cdc can be maintained constant. The last-stage DC/DC converter should be controlled to regulate the output charging current. Due to the boosting function of the boost DC/DC converter, the DC-link voltage Vdc is higher than the peak value of the input AC voltage vac. To keep such a voltage stable, a large electrolytic capacitor is required to be used for Cdc. Such large electrolytic capacitor has a short life span. The presence of the electrolytic capacitor also provides a means to adjust the power factor to approach unity.
It is advantageous if the power factor for a battery charger is correctable or adjustable to be close to unity without using a large electrolytic capacitor.